This invention relates to optical encoders.
Optical encoders are arranged to operate in either a transmission or a reflection mode. One such encoder is described, for example, in GB 2188144A which operates in a reflection mode. In this encoder, a disc carries a reflective track of spiral shape so that, as the disc rotates underneath a read head, the radiation reflected back varies in a way that can be detected to determine the angular position of the disc. Other forms of encoder have a plate with several parallel tracks having alternate transparent and opaque (or reflective and non-reflective) regions which differ in length between the tracks. Individual sensors in the read head detect whether the sensor is aligned with a transparent region or an opaque region in the respective track and thereby enable the position of the encoder to be determined to a resolution dependent on the length of the smallest region. Such an encoder is described, for example in GB Patent Application 9010248. Many different variations of encoder have been proposed.
One difficulty with optical encoders is that erroneous readings can be produced if, for example, dirt prevents transmission of radiation through what should be a transparent region, or if wear prevents reflection from what should be a reflective region.
Where the encoder is intended for use in critical applications, its correct operation can be confirmed by dividing the encoder plate into two or more separate and identical regions each having its own respective read head. The outputs from the different read heads can then be compared to confirm correct operation. This arrangement has the disadvantage of either limiting the range of displacement that can be measured, or increasing the size of the encoder plate.